The hepatitis C virus (HCV) is a leading cause of chronic liver disease worldwide (Boyer, N. et al. J Hepatol. 32:98-112, 2000) and may lead to hepatic fibrosis, cirrhosis and hepatocellular carcinoma (Cale, P., Gastroenterology Clin. Biol. 2009, 33, 958). A significant focus of current antiviral research is directed toward the development of improved methods of treatment of chronic HCV infections in humans (Di Besceglie, A. M. and Bacon, B. R., Scientific American, October: 80-85, (1999); Gordon, C. P., et al., J. Med. Chem. 2005, 48, 1-20; Maradpour, D.; et al., Nat. Rev. Micro. 2007, 5(6), 453-463). A number of HCV treatments are reviewed by Dymock et al. in Antiviral Chemistry & Chemotherapy, 11:2; 79-95 (2000).
Currently, there are primarily two antiviral compounds, ribavirin, a nucleoside analog, and interferon-alpha (α) (IFN), which are used for the treatment of chronic HCV infections in humans. Ribavirin alone is not effective in reducing viral RNA levels, has significant toxicity, and is known to induce anemia. The combination of IFN and ribavirin has been reported to be effective in the management of chronic hepatitis C (Scott, L. J., et al. Drugs 2002, 62, 507-556) but less than half the patients given this treatment show a persistent benefit. Other patent applications disclosing the use of nucleoside analogs to treat hepatitis C virus include WO 01/32153, WO 01/60315, WO 02/057425, WO 02/057287, WO 02/032920, WO 02/18404, WO 04/046331, WO2008/089105 and WO2008/141079 but additional treatments for HCV infections have not yet become available for patients. Therefore, drugs having improved antiviral and pharmacokinetic properties with enhanced activity against development of HCV resistance, improved oral bioavailability, greater efficacy, fewer undesirable side effects and extended effective half-life in vivo (De Francesco, R. et al. (2003) Antiviral Research 58:1-16) are urgently needed.
RNA-dependent RNA polymerase (RdRp) is one of the best studied targets for the development of novel HCV therapeutic agents. The NS5B polymerase is a target for inhibitors in early human clinical trials (Sommadossi, J., WO 01/90121 A2, US 2004/0006002 A1). These enzymes have been extensively characterized at the biochemical and structural level, with screening assays for identifying selective inhibitors (De Clercq, E. (2001) J. Pharmacol. Exp. Ther. 297:1-10; De Clercq, E. (2001) J. Clin. Virol. 22:73-89). Biochemical targets such as NS5B are important in developing HCV therapies since HCV does not replicate in the laboratory and there are difficulties in developing cell-based assays and preclinical animal systems.
Inhibition of viral replication by nucleosides has been extensively studied (De Clercq, E. (2001) J. Clin. Virol. 22:73-89) including nucleosides that inhibit RdRp. Generally, the antiviral activity of these nucleosides are attributed to the conversion of the nucleosides to their nucleoside triphosphates (NTPs) which act as inhibitors of DNA and RNA polymerases or as chain terminators following incorporation into the lengthening viral DNA or RNA strand. However, many NTPs lack adequate specificity for viral polymerases compared to host polymerases and, as a result, cause substantial toxicity. This has led to efforts to modify the core structures of nucleosides to achieve higher selectivity but many of the structural modifications have simultaneously compromised NTP production in the cells (Yamanaka, Antimicrob. Agents Chemother. 1999: 190-193).
The poor conversion of the nucleoside to NTP can often be attributed to the inability of nucleoside kinases to convert the nucleoside to the nucleoside 5′-monophosphate (NMP). NMP prodrugs have been used to bypass poor nucleoside kinase activity (Schultz, Bioorg. Med. Chem. 2003, 11, 885). Among these prodrugs, NMP phosphoramidates have been reported to increase intracellular concentrations of NTP compared to the nucleoside alone (McGuigan, J. Med. Chem. 1993, 36, 1048-1052). However, these NMP prodrugs are substrates for esterases and phosphodiesterases in the blood and other body tissues which can cleave the prodrug to a charged molecule or to the nucleoside, respectively. The charged molecule is then impermeable to the target organ or cell and the nucleoside is poorly phosphorylated intracellularly.
The development of a highly effective, non-toxic NMP prodrug is largely an unpredictable trial and error exercise requiring the balancing of the stability of the NMP prodrug in blood with the ability of the prodrug to reach a target organ or cell, be absorbed or actively taken up by the target cell, being efficiently cleaved to the NMP intracellularly and subsequently converted to a NTP that is selective for inhibiting the viral polymerase (Perrone, J. Med. Chem. 2007, 50, 1840-49; Gardelli, J. Med. Chem. 2009, 52, 5394-5407). For the case of an orally effective RdRp inhibitor for treating HCV infection, the NMP prodrug would need to be chemically stable to the conditions of the upper intestinal tract, be efficiently absorbed from the intestinal tract, survive the many esterases of the intestinal cells and blood, be efficiently extracted by the hepatocytes, and be cleaved to the NMP and subsequently converted to a NTP in hepatocytes that is specific for inhibiting the HCV NS5B polymerase.
Certain ribosides of the nucleobases pyrrolo[1,2-f][1,2,4]triazine, imidazo[1,5-f][1,2,4]triazine, imidazo[1,2-f][1,2,4]triazine, and [1,2,4]triazolo[4,3-f][1,2,4]triazine have been disclosed in Carbohydrate Research 2001, 331(1), 77-82; Nucleosides & Nucleotides (1996), 15(1-3), 793-807; Tetrahedron Letters (1994), 35(30), 5339-42; Heterocycles (1992), 34(3), 569-74; J. Chem. Soc. Perkin Trans. 1 1985, 3, 621-30; J. Chem. Soc. Perkin Trans. 1 1984, 2, 229-38; WO 2000056734; Organic Letters (2001), 3(6), 839-842; J. Chem. Soc. Perkin Trans. 1 1999, 20, 2929-2936; and J. Med. Chem. 1986, 29(11), 2231-5. However, these compounds have not been disclosed as useful for the treatment of HCV. Babu, Y. S., WO2008/089105 and WO2008/141079, discloses ribosides of pyrrolo[1,2-f][1,2,4]triazine nucleobases with antiviral, anti-HCV, and anti-RdRp activity.
Butler, et al., WO2009132135, disclose 1′ substituted ribosides and prodrugs comprising pyrrolo[1,2-f][1,2,4]triazine nucleobases which have anti-HCV and anti-RdRp activity but does not disclose species of the 3′-O-acylated derivatives of those ribosides or the expected properties of such derivatives.